Breast tumor kinase (Brk) a novel non-receptor protein tyrosine kinase cloned from a metastatic human breast tumor (1) is overexpressed in approximately two-thirds of human breast tumors but is absent from normal breast epithelial cells (2). Little is known about the role of Brk in breast cancer. Data from our lab has shown that shRNA knockdown of endogenous Brk in T47D breast cancer cells blocked ERK5 and p38 mitogen activated protein kinase (MAPK) activation in the presence of heregulin and EGF. Brk gene-silencing also induced T47D cell growth inhibition and reduced migration in Boyden chamber assays relative to control (non-specific shRNA) conditions. We have preliminarily shown that Brk kinase activity appears to be required for p38 MAPK and Erk5 phosphorylation and that Brk protein appears to have a long half-life. These results are preliminary but are beginning to approach areas of Brk related research that have not been addressed.